Preliminary evidence from our laboratory indicates that in vivo splenic irradiation (SRT) in tumor-bearing mice will significantly inhibit tumor growth and will lead to a significant rate of complete tumor regression. The SRT effect is presumably mediated by the elimination of non-specific spleen-seeking suppressor T cells (STC). These disappear from the spleens of mice following tumor regression. We have now studied SRT vis a vis splenectomy, a single versus multiple treatments, dose, timing and have compared SRT to cylophosphamide (Cy). Further study of SRT alone or in combination with Cy, hydrocortisone, dexamethasone, colchicine, or cimetidine are indicated. Characterization of the T cell or macrophage-mediated suppressor cell (SC) activity at various time points of tumor-bearing will be carried out using standard cell separation techniques of cellular immunology or using monoclonal antibody against T cell or macrophage markers. Characterization of spleen cells (SPC) from SRT non-responders and responders will be assessed using SC assays including the one way MLC, SC mediated antibody suppression, and in vivo abortion of tumor immunity by SPC transfer (reconstitution). The role of antigenicity, specificity, viral contamination, histology or tumor etc., will be examined in the MC1315 Balb/c system or in other carefully selected models. We anitcipate that these data may prove relevant to the ultimate design of human adjuvant therapy by redirecting conventional treatment, i.e., radiation, in a minimally toxic form of human anti-cancer treatment.